The invention relates to a paper machine fabric comprising at least two separate layers formed using at least two separate yarn systems: a yarn system forming the paper side and comprising machine direction and cross machine direction yarns and a yarn system forming the machine side and comprising machine direction and cross machine direction yarns, the yarn systems being arranged to form independent structures in the machine and cross machine directions of the fabric and the structures being bound together by means of binder yarns, a binder yarn being arranged to form part of the weave of a layer on the paper side surface and arranged to be interwoven with a layer of the machine side by being interwoven under at least one yarn in the machine side layer.
Conventional triple layer paper machine fabrics comprise two separate layers: a paper side layer and a machine side layer. The paper side layer and the machine side layer are interconnected mainly by means of a binder weft which serves as a binder yarn. Binding with a binder yarn usually takes place at every fourth top and bottom yarn pairs, i.e. relatively seldom. On the top side, the binding takes place over one top warp and on the bottom side, under one bottom warp. The binder yarn does not contribute to the forming of the paper side surface, but only to the binding of the layers. Consequently, the paper side layers and the machine side layers are not interconnected tightly enough. This causes xe2x80x9cinnerside wearxe2x80x9d in the fabric. Innerside wear refers to the wear caused by interlayer abrasion.
As a result of the interlayer wear mentioned above, the yarns wear down at the cross points of the binder yarn and warp yarns and later, as the fabric becomes looser, the yarns increasingly start moving with respect to each other, causing the intrinsic structures of the paper side and the machine side to wear down. The innerside wear of the fabric causes the binder yarn to start making markings on the surface of the paper because the fabric has lost its original thickness on its inner side while the binder yarn, in turn, has retained its original length. Highly increased innerside wear may also cause the layers to become detached from each other.
The binder yarn draws a warp yarn bound on the paper side slightly inwards. This dimpling causes marking. The binder yarn also forms an additional float stitch at this point in the structure. The fabric is thus denser at this point, and the water being removed from the paper web is not allowed to pass evenly through the fabric, which results in marking.
In a conventional triple layer fabric, the binder yarn on the paper side is preferably positioned slightly below the surface in order for the binder yarn not to cause marking. In order for the marking caused by the binding points to be as slight as possible, the binder yarn should be thinner than the top weft. Since the binder yarn cannot really be made thinner than it currently is (current diameter e.g. 0.13 mm), the top wefts cannot be made thinner than they currently are either (current diameter e.g. 0.15 mm), which means that the top weft density cannot be increased without decreasing permeability; consequently, paper fiber support remains low, particularly with high permeabilities.
Structures bound with binder yarn pairs are also known in the field. U.S. Patent Specifications U.S. Pat. Nos. 4,501,303, 5,967,195 and 5,826,627, for instance, describe techniques employed for binding structures using a binder yarn pair. In the structures bound using a binder yarn pair, instead of the binder yarn, it is the binder yarn pair which is responsible for binding the layers. A binder yarn pair comprises two adjacent binder yarns, one of the binder yarns establishing the paper side surface weave and the other simultaneously binding a paper side layer and a machine side layer together under one machine side bottom warp and vice versa. The zigzags of the binder yarn pair on the paper side surface establish a weft path similar to the top weft. The yarns of the binder yarn pair cross at a point where one binder yarn descends in the fabric from the paper side in order to bind the layers, while the other binder ascends in the fabric to form the surface of the paper side. This intersection prevents the binder yarns from moving into a straight line, which means that a paper side weft path formed by a binder yarn pair is not as straight as a weft path formed by an actual top weft.
The top weft positioned at both sides of the intersection presses the top warp yarns at the intersection downwards and, simultaneously, both yarns of the binder yarn pair descend into the fabric, not supporting the top warp yarns from below. Consequently, the intersections remain on a lower plane than the surface, which may cause marking. This is disclosed in U.S. Patent Specification U.S. Pat. No. 5,967,195.
In the structures bound using a binder yarn pair, the binder yarn pair comprises two yarns whereas one yarn constitutes the top weft. Typically, the top weft and the binder yarns have the same diameter. This means that the water drainage capacity at the binder yarn pair differs from that at the top weft, which, in the worst case, may appear in the form of marking in the paper. In order to ensure the water drainage capacity, the fabric must usually be coarser.
In the most common structures bound using a binder yarn pair, a bottom weft is provided at the top weft but no bottom weft is provided at the binder yarn pair, so the number of machine side bottom wefts is half the number of paper side weft paths, which means that there is little material to be worn down on the machine side, which results in poor wear resistance. If high wear resistance is to be achieved, density on the machine side should be the same, or almost the same, as on the paper side. If the machine side density and the paper side density are the same, weft density must be lowered in order to ensure the water drainage capacity, which results in poorer paper fiber support.
In the most common structures bound using a binder yarn pair, the yarns of the binder yarn pair are interwoven with every other top warp and the top weft is interwoven with every other top warp. Hence, every other top warp is interwoven in a different manner, remaining on a slightly different plane with respect to the surface. In paper grades most susceptible to marking this may appear in the form of marking in the warp direction.
An object of the invention is to provide a paper machine fabric to enable drawbacks of the prior art to be alleviated. This is achieved by a paper machine fabric of the invention, which is characterized in that the yarn system forming the paper side comprises a substitute yarn provided with a binder yarn woven on both sides thereof, and that on the paper side the substitute yarn is arranged to replenish the two yarn paths formed by the above-mentioned two binder yarns at points where the above-mentioned two binder yarns are interwoven with the machine side.
An advantage of the invention is e.g. the fact that the binder yarn not only binds the layers together but, in addition to the binding, the binder yarns also contributes to the forming of the paper side surface by being interwoven with several top warps, which enables binder yarns to be woven more densely than in the conventional triple layer fabrics. Consequently, the binding is tight, which prevents the layers from moving against each other. When the binder yarn contributes to the forming of the paper side surface, the marking caused by the binder yarn typical of the conventional triple layer fabrics is eliminated. The same property also enables excellent paper fiber support to be achieved in the structure. This advantage is important particularly with higher permeabilities where the number of cross machine direction yarns must be reduced in order to achieve sufficient permeability.
In the structures bound using a binder yarn pair, the top warp positioned at the intersection of the binder yarns is poorly supported from below, which means that at this point the top warp remains on a lower plane compared to the surrounding structure, the particular point causing marking in the paper. In the structure of the invention, on the paper side at the point where the binder yarn descends to the machine side in order to bind, the substitute weft forms a highly supportive bridge structure to lift the particular point up to the same plane as the surrounding structure, which results in no marking.
Since the fabric of the invention comprises no binder yarn pairs to tighten the structure, the bottom side weft density can be increased without the fabric becoming too dense, thus enabling more material to be provided on the machine side and more wear resistance to be achieved for the fabric.